1. Field of the Invention
This invention relates to a method of measurement of birefringence of an optical fiber and a measurement device, and to an optical fiber polarization mode dispersion measurement method and an optical fiber, and relates to techniques for the precise and simple measurement of the birefringence and polarization mode dispersion of an optical fiber along the length direction.
This application relates to and claims priority from Japanese Patent Application No. 2005-117030, filed on Apr. 14, 2005, and from Japanese Patent Application No. 2005-229263, filed on Aug. 8, 2005, the entire disclosures of which are incorporated herein by reference.
2. Description of the Related Art
In recent years, the faster transmission rates and longer transmission distances of optical communications have been accompanied by a need to reduce polarization mode dispersion (hereafter “PMD”) along transmission paths. PMD is dispersion which occurs due to group velocity differences among two eigenpolarization components which are orthogonal, propagating in an optical fiber (see Patent References 1 and 2 and Non-patent References 1 to 5).
There are two parameters determining the PMD. One is the magnitude of birefringence in the optical fiber; the other is the magnitude of the polarization mode coupling, which indicates changes in the optical fiber length direction of the birefringent axis in the optical fiber.
Specific factors determining PMD in an optical cable transmission path include ellipticity of the fiber core, asymmetry or the like of stresses occurring in the core, and other factors arising within the optical fiber, as well as asymmetry of stresses due to optical fiber bending in optical cable manufacturing processes and other factors arising from processes to produce optical cable. Hence in order to prevent worsening of PMD in optical cables due to factors within optical fibers, it is desirable that PMD arising from factors within the optical fiber be measured before processes to manufacture optical cable, and that optical fibers with poor PMD characteristics be excluded.
Optical fiber is normally wound on a bobbin for transport to the site of the process for optical cable manufacture. But an optical fiber wound on a bobbin is subject to bending and to birefringence caused by lateral pressure while wound on the bobbin; in addition, optical fibers will come into contact with each other or will be subjected to considerable torsion while being taken up on the bobbin, so that polarization mode coupling is induced. Consequently the PMD of an optical fiber wound on a bobbin does not coincide with the PMD arising from factors within the optical fiber.
Hence in order to measure PMD arising from factors within the optical fiber, a method is employed in which the optical fiber is removed from the bobbin and is wound with a diameter of from 20 cm to 100 cm, and by immersing the fiber in a liquid having a specific gravity close to that of the optical fiber, birefringence arising from lateral pressure and small-radius bending, as well as polarization mode coupling arising from contact between optical fibers, are eliminated, and the PMD is measured. This PMD measurement is for example described in Non-patent Reference 5.
As is stated in Non-patent Reference 4, PMD has statistical properties, and so measurements are attended by uncertainty. In order to reduce this uncertainty, methods may be used to increase the total PMD of the optical fiber for measurement, or expand the wavelengths for measurement, or apply perturbations to the optical fiber being measured and perform measurements a plurality of times.
Patent Reference 1: International Patent Publication No. WO 2004/010098
Patent Reference 2: International Patent Publication No. WO 2004/045113
Non-patent Reference 1: E. Chausse, N. Gisin, Ch. Zimmer, “POTDR, depolarization and detection of sections with large PMD”, OFMC '95.
Non-patent Reference 2: Tadao Tsuruta, Ouyou Kougaku 2, pp. 197-200, BAIFUKAN CO., LTD.
Non-patent Reference 3: R. C. Jones, “A new calculus for the treatment of optical systems VI. Experimental determination of the matrix”, JOSA, Vol. 37, pp. 110-112, 1947.
Non-patent Reference 4: N. Gisin, “How accurately can one measure a statistical quantity like polarization-mode dispersion”, PTL, Vol. 8, No. 12, pp. 1671-1673, December 1996.
Non-patent Reference 5: B. L. Heffner, “Automated measurement of polarization mode dispersion using Jones matrix eigenanalysis”, IEEE Photonics Tech. Lett. Vol. 4, No. 9, September 1992.
However, there are the following problems with the PMD measurement methods of the prior art.
In order to increase the total PMD of the optical fiber for measurement, the total length of the optical fiber for measurement must be made long if the optical fiber for measurement has a small PMD; but because an optical fiber used in PMD measurements in the free state cannot again be used as a product, this method requires a long optical fiber each time a measurement is performed, so that waste is substantial. Further, methods entailing expansion of the wavelengths for measurement are subject to constraints imposed by the operating wavelengths of the light source, and so there are limits to the use of such methods. And, methods requiring a plurality of measurements require time to perform measurements and are inefficient.
Next, another technology of the prior art, and problems with this technology, are described. Because there are large fluctuations in PMD depending on the preform and drawing conditions of the optical fiber, normally optical fibers manufactured under identical conditions exhibit substantially the same PMD value, but due to unanticipated causes, there are cases of partial worsening of the PMD, and so it is desirable that length-direction measurements be performed.
Methods of the prior art for longitudinal measurement of the birefringence and PMD include the methods described in Patent References 1 and 2. These methods involve measurement of the birefringence and PMD based on the amount of scattering in the OTDR waveform observed when a polarizer is placed between the OTDR and the optical fiber for measurement. However, these measurement methods are accompanied by a number of problems.
First, in methods of the prior art, the waveform amplitude differs depending on the incident polarization state and on the birefringence axis angle of the optical fiber, and so there is the problem that measurements cannot be performed accurately. For example, when the incident polarization is linear polarization, the amplitude is maximum when the angle between the direction of linear polarization and the birefringence axis is 45°, but when the two directions coincide, the amplitude is zero. This problem has a serious impact on the results of measurement of polarization mode dispersion using conventional methods.
Further, in methods of the prior art, scattering from the least-square approximating line is used as an index of the scattering in the OTDR waveform; to this end, averaging must be performed over a certain interval, and so it is inherently not possible to obtain a high resolution.
Moreover, a feature of methods of the prior art is the simple configuration obtained by using a general-purpose OTDR; but because the light source of a general-purpose OTDR has a broad spectral width from 5 nm to 20 nm, once a point with large PMD is traversed, a phenomenon occurs in which the polarization state of the pulse differs with the wavelength, and so the amplitude is averaged and becomes smaller; hence there is the problem that subsequent PMD measurements cannot be performed (see Non-patent Reference 1).